Why you weren't supposed to wear white after Labor Day.

Just for Enbrethiliel...
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"Never wear white after Labor Day". 
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The idea of a hard fast rule seems to be an American thing all right. The dictum most likely stems from the turn of the century when women wore a lot of black - a hold over from the Victorian era.  Yet in summertime, while on holiday and in the city, the girls wore pastels and white - often with white shoes if they were well to do.  White deflected the sun and was slightly cooler of course.  After everyone returned from the beach, the cape, the tropics, what have you, they all once again assumed the dull, drab dowager look of the era.  One would never wear white shoes with black anyway, although black shoes were worn with white.  Nevertheless avoiding white shoes was a practical solution to the dirty, muddy city streets in the fall, winter, and spring months.  White was simply impractical. 
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That said, the custom was clearly adopted from the aristocracy and upper class who loved rules.  Subsequently the emerging middle class and nouveau riche adopted the maxim - no white after Labor Day, and the dry goods stores (department stores) discovered a new merchandising strategy, as well as a marketing tool, thus introducing the novelty of the fall line... 
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European fashion never limited itself in this way - white was always fashionable.  Look at the Pope.
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So you see Sherman, anyone may wear white after Labor Day today...  fashion hasn't changed - it died.